Detergents have been widely used to solubilize tissue receptors. We have used detergent to solubilize insulin receptors from avian erythrocytes. The detergent solubilized insulin receptor retains all of the binding properties shown with cell and membrane preparations. In particular negative cooperativity was demonstrated and may be associated with a size change of the receptor. We are presently attempting to further characterize this size change. We are extending the study of insulin receptors to include other species along the phylogenetic line in order to determine the evolution of the receptor.